Drop extraneous dependencies from cudf conda recipe.

[bdice]

Description

This PR drops two dependencies from the cudf conda recipe.

• numba is only a run dependency and should not be listed in host.
• fastavro is only a test dependency. It is listed in dependencies.yaml but does not need to be listed in meta.yaml as a run dependency. This is already listed as a testing dependency in pyproject.toml.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[bdice]

/merge